Molecular and Kinetic Characteristics

Ornithine transcarbamylase has been isolated and purified partially or extensively from five mammalian livers: human(89–91), rat(93–96), beef,(97–99), pig(100) and dolphin (Stenella)(101) where OTC serves for detoxification of ammonia by the urea (ornithine) cycle. Urea synthesis and OTC activity occurs in a wide spectrum of vertebrate species for various reasons: amphibians after metamorphosis from the ammoniatelic state(102,103); turtles and tortoises for ammonia detoxification(103,104); sharks, rays, skates, Chimaera and coelacanths using urea as an osmolyte(104,105); a gobiid fish (Mugilogobius abei)(710) and an estivating lungfish for ammonia detoxification(106); toadfish using urea for nitrogen cycling between liver and gut(104); and for ammonia detoxification some teleost fish embryos and larvae (rainbow trout, cod(107), freshwater, air-breathing teleosts(108) and a tilapia fish living in an alkali lake(109). OTC activity has been detected in many species of mammals in addition to those noted above(102). Some hibernating mammals use the urea cycle to recycle nitrogen between liver and gut(104). Although birds excrete waste nitrogen as uric acid, the chicken kidney contains OTC for unknown reasons(110). Non-vertebrates which have a functioning urea cycle and OTC include earthworms and land planarians(111). A number of higher plant species as well as blue-green algae(112) contain OTC.